This invention relates to an electric discharge machining control apparatus which, during electric discharge machining, maintains optimum electric discharge machining conditions and performs control operations to maximize the electric discharge machining efficiency.
In an electric discharge machining operation, in order to maintain the electric discharge machining conditions stable and to improve the electric discharge machining efficiency, it is necessary to suitably control a pause time, electric discharge time, servo reference voltage, and spindle feed speed (spindle feed speed gain) during machining.
FIG. 1 is a block diagram showing a conventional electric discharge machining control apparatus according to a pause time control system.
In FIG. 1, reference numeral 1 designates an electric discharge machining process including electric discharge phenomena; 2, status data of electric discharge machining process; 3, a machining power source; 4, a status detector for detecting the status data 2; 5, the detection value of the status detector; 6, a pause time setting unit; 7, the instruction value provided by the setting unit 6; 8, a pause time controller for controlling a pause time in response to the instruction value provided by the pause time setting unit 6 and to the detection value 5 of the status data of electric discharge machining process outputted by the status detector 4; and 9, the pause time data provided by the pause time controller 8.
The operation of the conventional electric discharge machining control apparatus thus organized will be described.
The operator sets a pause time, one of the electric discharge machining conditions, with the pause time setting unit b, to perform an electric discharge machining operation. During discharge machining, the gap between a machining electrode and a workpiece is generally narrow, of the order of 10 to 50 .mu.m. As the machining advances, waste material such as small particles is formed and caught in the gap, so that electric discharges take place with the waste material with the result that secondary electric discharge or abnormal electric discharge is liable to occur. This is due to the fact that the quantity, of waste material formed is larger than the quantity of waste material which can be removed. This difficulty is eliminated as follows: When such abnormal condition is detected, the pause time is increased according to the abnormal condition thus detected, thereby to prevent the accumulation of waste material in the discharge gap.
The status detector 4 detects the status data 2 of process, and applies the occurrence of abnormal electric discharge, as the detection value 5, to the pause time controller 8. In response to the detection value 5 provided by the status detector 4 and the instruction value provided by the pause time setting unit 6, the pause time controller 8 controls the pause time. The pause time data 9 is applied to the machining power source 3.
As is apparent from the above description, controlling the pause time is essential for maintaining the electric discharge machining conditions stable. However, the method lowers the machining efficiency; that is, it wastes the time to be used for the machining operation. Thus, in order to improve the machining efficiency, it is essential to control the pause time effectively. For this purpose, it is necessary to determine pause increasing or decreasing conditions according to the pulse conditions of the machining power source, the configuration of the machining electrode, and the materials of the machining electrode and the workpiece in combination. This determination depends greatly on the know how of a skilled operator. Such a skilled operator monitors how unstable the electric discharge machining conditions are, to adjust the pause time.
FIG. 2 is a block diagram showing a conventional electric discharge machining control apparatus according to an electric discharge duration control system. In FIG. 2, reference numeral 1 designates an electric discharge machining process including an electric discharge phenomenon; 2, status data of electric discharge machining process; 3, a machining power source; 4, a status detector for detecting the status data 2; 10, an electric discharge duration setting unit 10 for setting an electric discharge duration; 11, an instruction value provided by the electric discharge duration setting unit; 12, an electric discharge duration controller for controlling the electric discharge duration according to an instruction value provided by the electric discharge duration setting unit 10 and the detection value 5 of the status data of electric discharge machining process which is provided by the status detector 4; 13, electric discharge duration data provided by the controller 12.
The operation of the control apparatus thus organized will be described.
In an electric discharge machining operation using an electrode of graphite material, the operator sets an electric discharge duration time, one of the machining conditions, with the electric discharge duration setting unit 10. Because of electric discharge machining characteristics, as the electric discharge duration time increases, the amount of consumption of the electrode is decreased, and the machining accuracy is increased; however, if the electric discharge duration time is increased excessively, then the occurrence of electric discharges concentrates at a corner of the electrode, as a result of which secondary electric discharge or abnormal electric discharge is liable to occur. Furthermore, in the electric discharge machining operation using the graphite electrode, as shown in FIG. 3 protrusions 41 is formed at a corner of the electrode 40, thus making it impossible to continue the machining operation. In order to eliminate this difficulty the following method is employed: When the abnormal electric discharge is detected, the electric discharge duration time is decreased so that the protrusions formed at the corner of the electrode are consumed. Under this condition, the machining operation is continued until it becomes stable. After all the protrusions have been removed, the machining conditions are restored. By repeatedly carrying out the above-described operation, not only the consumption of the electrode but also the machining time can be minimized. The status detector 4 for detecting abnormal electric discharges operates to detect, for instance, the amplitude in movement of the electrode or the upward movement of the electrode from the machining deepest point during electric discharge machining.
The status detector 4 detects the status data 2 of process, and applies the occurrence of abnormal electric discharge, as the detection value 5, to the electric discharge duration controller 12. The latter 12 controls an electric discharge duration according to the instruction value provided by the electric discharge duration setting unit 10 and the status data of electric discharge machining process provided by the status detector 4, to provide electric discharge duration data 13, which is applied to the machining power source 3.
As is apparent from the above description, controlling the electric discharge duration is essential for stabilization of a electric discharge machining operation; however, it is not always preferable in terms of machining accuracy; that is, in order to improve the machining efficiency, it is essential to control the electric discharge duration effectively. For this purpose, it is necessary to determine electric discharge duration increasing or decreasing conditions according to the pulse conditions of the machining power source, the configuration of the machining electrode, and the materials of the machining electrode and the workpiece in combination. In general, this determination depends greatly on the know how of a skilled operator. Such a skilled operator monitors the instability of the electric discharge machining conditions, and adjusts the electric discharge duration time.
FIG. 4 is a block diagram showing a conventional electric discharge machining control apparatus of servo reference voltage control system. In FIG. 4, reference numeral 1 designates an electric discharge machining process including electric discharge phenomenon; 2, the status data of electric discharge machining process; 14, an electrode control system; 15, a distance signal corresponding to the machining gap between a workpiece and a machining electrode which is adjusted by the electrode control system; 4, a status detector for detecting the status data 2; 5, a detection value provided by the status detector; 16, a servo reference voltage setting unit for setting a servo reference voltage for an electric discharge machining operation; 17, an instruction value (Vref) provided by the unit 16; and 18, an arithmetic unit for obtaining the difference 18a between the instruction value (Vref) and the detection value 5. The difference 18a is applied to the electrode control system 14. The system 14 adjusts the machining gap distance signal 15 so that the difference 18a is zeroed.
FIGS. 5 and 6 show the relationships between the servo reference voltage instruction value Vref and inter-electrode voltage waveforms. In these figures, reference character V.sub.M designates an inter-electrode average voltage, and T.sub.M, a no-load time. In the case of FIG. 5, the servo reference voltage instruction value Vref is high, while in the case of FIG. 6, it is low. In the case where the servo reference voltage instruction value Vref is high as shown in FIG. 6, the inter-electrode average voltage V.sub.M is high, and the no-load time T.sub.M is long; that is, an electric discharge standby time from the application of an inter-electrode voltage is long, as a result of which the distance 15 of the machining gap is large. On the other hand, in the case where the servo reference voltage instruction value Vref is low as shown in FIG. 5, the inter-electrode average voltage V.sub.M is low, and the no-load time T.sub.M is short; that is, an electric discharge standby time from the application of an inter-electrode voltage is short, as a result of which the distance 15 of the machining gap is small. Thus, when the servo reference voltage instruction value Vref is increased, then the distance 15 of the machining gap is increased, as a result of which the waste material formed during machining can be removed with ease, and the machining operation becomes stable accordingly; however, the machining speed is decreased. When, on the other hand, the servo reference voltage instruction value Vref is decreased, then the distance 15 of the machining gap is decreased, it becomes rather difficult to remove the waste material, and the machining operation is rather unstable; however, the machining speed is increased.
In general, before the start of an electric discharge machining operation, the operator determines the servo reference voltage value according machining contents such as a machining depth, an electrode configuration, a machining solution supplying method, and the materials of an electrode and a workpiece, and set is with the servo reference voltage setting unit 16.
As is apparent from the above description, the servo reference voltage value is essential for maintaining the electric discharge machining conditions stable and for increasing the machining speed. Thus, it is important to set the servo reference voltage to a most suitable value. That is, the servo reference voltage value should be determined according to variations in machining depth, machining power source pulse conditions, an electrode area confronting a workpiece, a machining solution supplying method, and the materials of an electrode and a workpiece in combination. In general, this determination depends greatly on the know how of a skilled operator. He monitors the instability of the electric discharge machining conditions, to adjust the servo reference duration time.
FIG. 7 is a block diagram showing a conventional electric discharge machining control apparatus of speed gain control system. In FIG. 7, reference numeral 1 designates an electric discharge machining process including a electric discharge phenomenon; 2, the status data of electric discharge machining process; 4, a status detector for detecting the status data of the process; 5, a detection value of the status data of the process; 43, an electrode control system; 43a, the response speed of the electrode controlled by the electrode control system; 44, a speed gain setting unit for setting a spindle feed speed gain for a machining operation; 44, a set value provided by the unit 44a; 45, an amplifier for applying a speed gain to a spindle feed speed instruction value; 45a, a feed speed instruction value amplified with the speed gain; 46, an arithmetic unit for performing an arithmetic operation with the detection value 4 of the status data fed back to the servo voltage instruction value; and 46a, the output of the arithmetic unit 46.
The operation of the electric discharge machining control apparatus thus organized will be described.
Before an electric discharge machining operation, a speed gain, one of the machining conditions, is set with the speed gain setting unit 44. In the electric discharge machining operation, the reference servo voltage instruction value, and the detection value of the status data which is detected by the status detector 4 are applied to the arithmetic unit 46, the output 46a of which is applied to the amplifier 45. In the amplifier 45, the input A (i.e., the output 46a of the arithmetic unit 46) is amplified with a gain K set by the speed gain setting unit 44. The output 45a (A.sub.K =K A ) of the amplifier is applied to the electron control system 43. The latter 43 changes an electrode forward/backward response speed 43a according the output 45a of the amplifier, thereby to control the electrode in the machining process 1.
FIGS. 8 and 9 show the relationships between an inter-electrode voltage waveform and a gain set value 44a. In these figures, reference character T.sub.M designates a no-load time. When the speed gain is low, as shown in FIG. 6 the no-load time T.sub.M changes. This is because, when the speed gain is low, the electrode response speed is decreased, as a result of which it becomes impossible for the electrode to follow the inter-electrode variation in the preceding electric discharge machining process, and correction of the variation takes time. Thus, the machining speed is decreased.
In the case where the speed gain is high, as shown in FIG. 9 the electrode response speed is high, and the electrode can follow the inter-electrode variation with ease. Accordingly, the time required for eliminating the inter-electrode variation caused in the preceding electric discharge machining process is short. That is, as shown in FIG. 9, the no-load time T.sub.M, or standby time until electric discharges are induced, is reduced, so that the machining speed is increased. However, if the speed gain is too high, then the electrode follows the inter-electrode variation excessively; that is, it is oscillated, thus making it impossible to perform the machining operation.
At the start of an electrode discharge machining operation, the operator sets the speed gain with the speed gain setting unit according to a machining depth, an electrode configuration, a machining solution supplying method, and the materials of an electrode and a workpiece in combination.
As is apparent from the above-description, setting the speed gain is essential for maintaining the electric discharge machining conditions stable and for increasing the machining speed. Thus, it is important to set the speed gain to its most suitable value in order to improve the machining efficiency. That is, the speed gain should be determined according to variations in machining depth, machining power source pulse conditions, an electrode area confronting a workpiece, a machining solution supplying method, and the materials of an electrode and a workpiece in combination. In general, this determination depends greatly on the know how of a skilled operator. He monitors the instability of the electric discharge machining conditions, to adjust the speed gain.
The conventional electric discharge machining control apparatus is constructed as described above. Hence, in the case where a pause time, discharge duration time, servo reference voltage, and speed gain are set according to the know how of a skilled person for best pause time control, best discharge duration time control, best servo, reference voltage control, and best speed gain control, it is difficult to write the quantitative, fuzzy expressions included in the know how as methods of suitably controlling a pause time, discharge duration time, servo reference voltage, and spindle feed speed, and accordingly the resultant methods include personal errors. In the case where the pause time, discharge duration time, servo reference voltage, and speed gain are controlled automatically, not by the skilled person, according to the instability of electric discharge machining conditions, it is difficult to correctly write the criterions given by the skilled person. Thus, the conventional electric discharge machining control apparatus has problems to be solved for improvement of the electric discharge machining efficiency.